Mueller Did Not Get to the Bottom of This

One of the most surprising aspects of the Mueller report, to me, was that Mueller openly admitted that he did not get to the bottom of the Russia conspiracy probe after two years of investigation. I would have thought that, after seemingly flipping Michael Flynn, Rick Gates, and Michael Cohen, Mueller would have sought to compel Donald Jr. and/or Jared Kushner to testify, getting to the truth of the matter of who, if anyone, was in charge of coordinating with the Russians and whether there was a clear quid pro quo. He did not. First, he did not compel anyone from the Trump family to testify. Second, he never interviewed the President. Third, many of the subjects of the investigation lied and deleted evidence. Therefore, all he could report was that there were a lot of potentially shady contacts and dealings with the Russians. A lot of suspicious activity that could have included a quid pro quo type of agreement between the Trump campaign and the Russians, but no irrefutable evidence that would have warranted prosecution. Conclusion: looks like something dirty is going on, but cannot exactly say. That sets up the obstruction part of the probe, which is a lot more clear cut. And raises the question: if the Trump world was engaging in this much obstruction, wasn't there something to the Russia investigation that Mueller failed to turn up?